A known example of a process of purifying wastewater containing contaminants of the prior art consisted of removing heavy metal ions present in wastewater by reducing those metal ions by adding a reducing agent to the wastewater, and iron powder and so forth was used for the reducing agent.
For example, a process is described in Japanese Unexamined Patent Application, First Publication No. H9-262592 in which a layer packed with iron particles is formed in a column-shaped tank, and wastewater is passed through this iron particle packed layer to remove heavy metals by adsorbing them onto the surface of the iron particles. However, in processes that use iron powder for the reducing agent, since their reducing power decreases rapidly as a result of the surface reaction being impaired when heavy metals are adsorbed onto the surface of the iron particles, it is necessary to replace the iron powder at short intervals, thereby resulting in the problem of a large maintenance burden. Moreover, post-treatment is required under acidic conditions in particular due to the generation of hydrogen gas and divalent iron. In addition, the packed layer becomes extraordinarily heavy due to the use of a large amount of iron powder, thereby placing a large burden on the apparatus structure as well.
In addition, selenium present in wastewater is subjected to strict discharge standards as an environmental contaminant. Normally, selenium is present in wastewater in the form of selenite ions (SeO32−) (tetravalent selenium) and selenate ions (SeO42−) (hexavalent selenium). Examples of known processes for removing this selenium include: (i) a process in which a trivalent iron compound such as ferric hydroxide is added to co-precipitate the selenium by adsorbing it to a precipitate by taking advantage of its aggregating action, (ii) a process in which barium or lead and so forth is added to form a refractory selenate precipitate, (iii) a process in which selenium is removed by adsorbing using an ion exchange resin, and (iv) biological treatment processes.
However, since co-precipitation by barium or lead is susceptible to the effects of other ions present, it is necessary to increase the amount added, and a burden is placed on post-treatment since barium and lead are also heavy metals. In addition, processes using an ion exchange resin have the problem of their removal effects decreasing dramatically in the presence of sulfate ions and so forth. Moreover, biological treatment processes have a long treatment time.
On the other hand, processes using trivalent iron compounds have hardly any effects on hexavalent selenium. Therefore, a process has been proposed that uses ferrous salt (divalent iron). This process promotes the precipitation of selenium by reducing hexavalent selenium to tetravalent selenium using the reducing power of ferrous iron.
For example, Japanese Unexamined Patent Application, First Publication No. H08-267076 describes a treatment process in which divalent iron ions are added to selenium-containing wastewater followed by the addition of an alkaline compound in an environment isolated from air while heating and maintaining the liquid temperature to 30° C. or higher to form a selenium precipitate.
Japanese Unexamined Patent Application, First Publication No. 2002-326090 describes a treatment process comprising a first step, in which hydroxides of heavy metals are precipitated by adding an alkaline compound to selenium-containing wastewater, a second step, in which an inert gas is introduced into this treatment liquid to remove dissolved oxygen followed by adding ferrous salt in the alkaline range to reduce and precipitate the selenium, and a third step, in which air is blown into this treatment liquid to precipitate heavy metals remaining in the liquid by incorporating in an iron-containing precipitate.
Japanese Unexamined Patent Application, First Publication No. 2001-9467 describes a treatment process which, on the one hand, forms a selenium-containing precipitate by adding ferrous hydroxide to selenium-containing wastewater and then adding an alkaline compound, while on the other hand, enhances treatment efficiency by circulating a portion of this sludge to a reaction tank following addition of an alkaline compound.
However, it is difficult to lower the selenium concentration in wastewater to 0.01 mg/L or lower with the aforementioned treatment processes of the prior art. In addition, in processes simply involving the addition of ferrous hydroxide, the treatment process is complicated due to the need to preliminarily remove dissolved oxygen in the wastewater since oxygen in the wastewater competes for reaction with ferrous ions with the selenium. Moreover, since precipitates of ferrous hydroxide have a high moisture content and a large apparent density, they place a large burden on slurry treatment if used in that form.
Furthermore, although processes are known in which a portion of the formed precipitate is circulated to a reaction tank, since the consolidating effects of precipitation are still low if the formed precipitate is merely circulated, a burden is placed on post-treatment. Moreover, since many treatment processes of the prior art use iron ferrite by heat-treating ferrous hydroxide, in addition to the treatment process becoming complex, there is also the problem of increased heating costs.
In addition, a treatment process for removing heavy metals from wastewater in which ferrous iron ions and so forth are added to wastewater containing heavy metals, iron ferrite or pseudo-iron ferrite is formed by adjusting the pH to 5 or higher, and the formed ferrite sludge is then separated into solid and liquid together with circulating the sludge by returning a portion to a reaction tank (Japanese Unexamined Patent Application, First Publication No. 2001-321781).
This process focuses on the fact that ferrite sludge (FeO.Fe2O3) contains ferrous iron and ferric iron, and forms a precipitate by utilizing the fact that the presence of both ferrous iron and ferric iron more easily forms a ferrite sludge than ferrous iron alone. However, since the ferrite sludge of this treatment process has low reducing power, there are limitations on its heavy metal removal effects even if returned to a reaction tank.
On the other hand, in a wastewater treatment process in which a sludge is precipitated by adding an alkaline compound to wastewater containing heavy metals followed by separation of this sludge, the alkaline compound is not added directly to the heavy metal wastewater, but rather is only added to a portion of the separated sludge, after which this alkaline sludge is returned to a reaction tank (Japanese Examined Patent Application, Second Publication No. S61-156, Japanese Unexamined Patent Application, First Publication No. H05-57292 (Japanese Patent No. 2910346)). However, it is difficult for the alkaline sludge alone to lower heavy metal levels to equal to or below environmental standard values.
In addition, magnetic separation means are known as means for efficiently separating heavy metal aggregates or heavy metal precipitates when removing heavy metals contained in wastewater by precipitation or aggregation.
Japanese Unexamined Patent Application, First Publication No. 2000-117142 describes a means that aggregates heavy metal ions in waste liquid, and a separation means that uses a magnetic filter to entrap particles present in a waste liquid by forming a strong magnetic field with a superconducting solenoid magnet.
Japanese Unexamined Patent Application, First Publication No. 2001-321781 described a treatment process in which ferrite sludge is formed by adding ferrous iron ions to heavy metal wastewater followed by separating with a thickener or magnetic separator and so forth.
Japanese Unexamined Patent Application, First Publication No. 2001-259657 describes a treatment process in which magnetite particles and so forth are added to form aggregates having increased magnetism followed by magnetic separation, that is used when aggregating and/or precipitating phosphorous and heavy metals by the aggregation/precipitation and ferrite methods.
However, there are limitations on the magnetic separation effects of magnetic separation employed in the aforementioned treatment processes of the prior art since magnetic fields are applied statically in all of these processes. Since precipitates of heavy metals contained in wastewater are particularly diverse depending on the types and precipitated states of the heavy metals, there is the problem of being unable to obtain adequate separation effects simply by statically applying a fixed magnetic field.